1. Field of the Invention
The invention relates to separators for use in a fuel cell stack, and more particularly to supply of reaction gas to fuel cells via the separators.
2. Description of the Related Art
A fuel cell, for example, a solid polymer fuel cell, includes two electrodes (i.e., an oxygen electrode and a fuel electrode) that are opposed to each other, and an electrolyte membrane sandwiched between the two electrodes. By supplying fuel gas containing hydrogen and oxidant gas containing oxygen to the fuel electrode and the oxygen electrode, respectively, reactions as expressed by the following formulae (1) and (2) occur in the fuel cell so as to directly convert chemical energy of the substances into electric energy.On the cathode (oxygen electrode) side: 2H++2e−+(½)O2→H2  (1)On the anode (fuel electrode) side: H2→2H++2e−  (2)
As a typical structure of the fuel cells as described above, a stacked structure has been developed in which generally planar, membrane electrode assemblies (MEA) and separators are stacked or laminated together and are fastened or joined together in the stacking direction.
As one type of the separators, a fuel cell separator having a three-layer structure is known which consists of an anode-side plate, a cathode-side plate, and an intermediate plate sandwiched between the anode-side plate and the cathode-side plate. One example of this type of separator is disclosed in, for example, Japanese Laid-open Patent Publication No. 2004-6104. The separator of the three-layer structure as disclosed in this publication has reactant gas manifolds that penetrate the three plates, gas transfer channels formed in the intermediate plate, and through-holes in the form of slits formed in opposite end portions of reactant gas channels formed by half-etching in the anode-side plate and the cathode-side plate. In this arrangement, reactant gas is distributed from the gas transfer channels to the reactant gas channels via the slit-like through-holes.
In the known separator structure as described above, however, the gas transfer channels formed on the reactant-gas supply side have the same configuration and arrangement as the gas transfer channels formed on the reactant-gas exhaust side, and no consideration is given to the case where the gas transfer channels are employed in a separator (hereinafter called “flat separator”) in which no reactant gas channels are provided in the anode-side plate and the cathode-side plate. In the case of the flat separator, the use of the gas transfer channels as described above may result in reduced efficiency or uniformity with which the reactant gases are supplied to the respective electrodes. Namely, the flat separator is not provided with reactant gas channels in the anode-side plate and the cathode-side plate, and is thus not able to control distribution of the reactant, gas by means of the reactant gas channels. Accordingly, a technology for improving the uniformity of distribution of the reactant gas has been desired with regard to a structure (e.g., channel arrangement) on the supply side of the reactant gas and a structure on the exhaust side of the reactant gas. In addition to the uniform distribution of the reactant gas, it has been desired to discharge water as a reaction product to the outside of the fuel cell stack with improved efficiency.